Sulphur dioxide is a gas which forms in the oxidation of sulphur-containing materials, such as coal, oil, natural gas, industrial and domestic waste, peat, etc. Sulphur dioxide can also form as a residual product in chemical processes, for instance metallurgical processes. Normally, it is not permitted to emit large amounts of sulphur dioxide into the atmosphere, which means that some kind of cleaning is necessary. One example of this is flue gas cleaning in power plants and other combustion plants. The flue gas that forms in combustion in such plants is usually cleaned, among other things, by absorption of sulphur dioxide into an absorption liquid. The absorption liquid can, for instance, contain water and one or more of the substances lime, limestone, dolomite, sodium hydroxide solution and similar substances, which are suitable for the absorption of sulphur dioxide. The flue gases can, for instance, be cleaned in a spray tower, as disclosed for instance in EP 0 162 536, or by means of a perforated tray, as disclosed for instance in U.S. Pat. No. 5,246,471. However, these devices for cleaning gases, in particular flue gases, from sulphur dioxide have been found to require a great deal of energy as large amounts of absorption liquid is pumped at a relatively high pressure.
U.S. Pat. No. 4,099,925, U.S. Pat. No. 5,660,616, U.S. Pat. No. 4,239,515 and WO 96/00122 describe cleaning apparatuses with low energy consumption. The flue gas is conveyed upwards through an apertured plate, on which a flowing layer of an absorption liquid is provided.
If the flue gas is not saturated with water vapour, water will evaporate from the absorption liquid and be added to the flue gas during the cleaning process. It has been found that this evaporation partially takes place when the flue gas passes through the apertured plate. One problem is that substances, such as lime, limestone, gypsum, calcium sulphite, sodium sulphate, etc, which are dissolved or suspended in the absorption liquid, tend to be evaporated and precipitated on the underside of the apertured plate and in the holes of the apertured plate. This increases the pressure drop across the apertured plate and makes the pressure drop vary over the area of the apertured plate. This results in increased energy consumption due to the increased pressure drop and in reduced absorption of sulphur dioxide due to the uneven distribution of flue gas in the layer of absorption liquid on the apertured plate. The prior-art solution to this problem is to arrange, before the cleaning apparatus with the apertured plate, a cooler in the form of a separate spray tower, for instance of the type disclosed in U.S. Pat. No. 5,753,012. In the separate spray tower, into which the flue gas is first introduced, an aqueous liquid is injected at a ratio (also called L/G) of the flow of liquid to the flow of flue gas of typically about 0.2–1 litre of liquid/m3 of flue gas and at such a high pressure that the liquid is atomised and saturates the flue gas with water vapour. After being saturated with water vapour, the flue gas can be passed through the apertured plate without the risk of solids being precipitated. A separate spray tower is however a complicated and energy-consuming solution, which comprises pumps, pipes, tanks, control systems and a separate tower. In addition, when using such a spray tower semi-dry particles can form, which adhere to the underside of the apertured plate. It is therefore sometimes necessary to arrange a system for intermittent washing of the underside of the apertured plate.